The demand for transmission capacity enhancement in fiber networks and for low cost per bit for high bit-rate systems requires continuous improvement in single channel bit-rates. Electrical time division multiplexed (ETDM) systems at bit-rates of 40 Gb/second per channel are commercially available, and optical time division multiplexed (OTDM) systems offering bit-rates of 160 Gb/s per channel have been demonstrated.
Aside from other transmission impairments like chromatic dispersion or non-linear pulse propagation, polarization mode dispersion (PMD) remains an obstacle to be overcome before these ultra high time division multiplexed (TDM) systems can be implemented on a significant number of installed fibers. Several different PMD compensation and mitigation techniques have been proposed and partially demonstrated. They can be generally categorized according to concepts based on the electrical or optical domains of the system. Electrical mitigation techniques possess advantages in terms of cost and size. However, the performance improvements from electrical techniques are generally not as great as those achievable with optical compensation techniques.
For example, optical techniques can be designed to be bit-rate and format independent. The measurement of the polarized components of an optical signal may be used for implementing PMD compensation using optical techniques.
Aside from PMD compensation, simple polarization monitoring will also be an important feature in future networks employing polarization multiplexing. Furthermore, polarization measurement is required as well for signal polarization adaptation to polarization sensitive receivers.